Until 1947 all the states (except Afghanistan) were
either ruled by or dominated by the British Indian Empire.

Pakistan, including at that time Bangladesh, was split off
to give the Muslims a homeland. India has a majority of Hindus
though is nominally a secular (religiously neutral) state. Sri
Lanka had never been ruled as part of India. Nepal had been a
British Protectorate and, although a Hindu kingdom, did not join
India. Sri Lanka has a Buddhist majority but a large Hindu minority.
Bangladesh became independent after a war with what was then
West Pakistan.

At present (2006) India acts as the regional Great Power,
having intervened in Bangladesh and Sri Lanka and bringing pressure
to bear on Nepal and Bhutan. India and Pakistan are usually in
a state of disagreement or Cold War, three times resulting in
actual conflict, usually over Kashmir.

There is a regional organization (all except Afghanistan):
South Asian Association for Regional Cooperation. At present
it acts for consultation. Both India and Pakistan count as nuclear
powers.